When Einstein published his General Theory of Relativity, surely even he could not be fully aware of the profound effect it would have on the scientific community.

The theory of General Relativity consists of only a few fundamental building blocks, the basics of which may be understood by first acknowledging just a few key principles:

• The Principle of Equivalence states that inertial mass is identical to gravitational mass, meaning that one cannot tell the difference between mass caused by motion and mass caused by gravity.

• As a result of the Principle of Equivalence, gravity must in some manner be caused by inertia.

• Special Relativity showed that neither space nor time is constant – that they expand or contract as a result of an object’s motion.

• One way of explaining the Principle of Equivalence using the flexibility of spacetime is by assuming that the shape of the universe as a whole is not flat, but is curved by the mass of the matter contained within.
• This being assumed, gravity is caused by an object being caught up in the curved space-time surrounding a massive body.

In essence, these are the basic principles which led Einstein to a complete theory of General Relativity, though a full understanding of them requires many years of difficult mathematical study and a comprehensive knowledge of non-Euclidean geometry, which admittedly not very many people have time for.

Tests of General Relativity

One of the more remarkable elements of General Relativity as Einstein defined it was that along with it came several tests prepared in advance. Einstein personally came up with several ways which experimental physicists could attempt to verify his theory – and while none of them in and of themselves could possibly prove the theory (for by its nature, a theory is unprovable), they would certainly help to quell doubters and show that it is at the very least probable. And the reverse of this was true as well – if the results of these tests turned out negative, they would be able to show that Einstein was in error.

The first and perhaps most famous of these tests is that of gravity’s ability to bend light. According to the Principle of Equivalence and the theory of curved spacetime, rays of light should have their paths slightly altered when passing near a massive object (and, obviously, the more massive the object the more dramatic this alteration).

In 1919, Sir Arthur Eddington led a team of researchers which successfully observed the light of stars passing by the sun during an eclipse (for this is the only condition in which such a measurement is possible). Their results made great waves in the scientific community, and helped to show that Einstein’s theory was no mere flash in the pan.

A second test was based on experiments which already existed, and was therefore more mathematical in nature. This had to do with the orbit of the planet Mercury around the sun; specifically regarding the planet’s perihelion shift.

As the planets orbit the sun, there are certain points in the orbit when they are closest to the sun. This point is known as the perihelion. Over time, this closest point tends to shift in position, little by little. The reasons for this are many, but using only Isaac Newton’s theory of gravity, astronomers had been able to predict the perihelion shift of every planet with near perfect accuracy – every planet, that is, except for Mercury.

The problem, Einstein realized, is that Newton’s equation wasn’t able to account for the dramatic curvature of space-time which occurred near the sun, and which, of the planets, only Mercury was close enough to truly be affected by. After applying his own equations to the problem, however, Einstein realized that General Relativity gave a much more agreeable answer to this problem.

Einstein’s theory, therefore, was not only able to explain all previously understood phenomenon, it was able to finally solve previously unexplained occurrences – one of the fundamental tests of a good scientific theory.

Consequences of the Theory

As a result of General Relativity, scientists have offered an entirely new way of looking at the universe around them, and entire branches of science that would have made no sense before the theory, have become major fields of research.

Physicists exploring the origins and future of the universe as a whole (from the big bang onward), have begun to explore how the shape of the universe has changed over time as a result of its collective mass. While this may seem to be a rather unimportant consideration, it really is rather interesting. For example, scientists have realized that a universe with a certain shape and a certain amount of mass will continue to expand forever without stopping. A universe of another shape will eventually stop expanding, and will be pulled back inward by the gravity of its own mass (this is often called the “Big Crunch”). Still another shape will one day stabilize and remain somewhat constant for eternity.

Still another result of General Relativity, a hypothetical phenomenon known as the Casimir Effect, has been theorized to make time travel possible by way of utilizing the curved spacetime of General Relativity; and similarly the use of wormholes might make faster-than-light travel possible – an important loophole which might one day be used to bypass those pesky roadblocks Einstein theorized in Special Relativity.

Relativity Today

While all of this may seem wonderful to the layman, to scientists it is simply not that easy. Both General and Special relativity are still being tested, and they are still being questioned. While neither theory will ever be fully proven, they both have become experimentally very well-tested, and have proven to be remarkably successful at predicting the behaviors of our universe. The question remains, however, as to whether relativity can be classified as “final” or whether it is still simply another stepping stone on the path to a theory that can explain even more.

Questions like these are what make physics so very interesting.

References:

Weinberg, S. (1992). Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature. New York, NY: Vintage Books.

Moring, G. F. (2004). The Complete Idiot's Guide to Understanding Einstein. New York, NY: Alpha Books.

Hawking, S. (1988). A Brief History of Time. New York, NY: Bantam Books.

Einstein, A. (1961). Relativity: The Special and the General Theory - A clear Explanation that Anyone can Understand. New York, NY: Random House.